1. Field of the Invention
This invention relates to polyol ester lubricant base stocks, which can also serve as complete lubricants in some cases, refrigerant working fluids including lubricants according to the invention along with primary heat transfer fluids, as well as methods for using these materials in refrigeration equipment. The lubricants and lubricant base stocks are generally suitable for use with most or all halocarbon refrigerants and are particularly suitable for use with substantially chlorine-free, fluoro-group containing heat transfer fluids such as pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluoroethane, and 1,1,1,2-tetrafluoroethane.
2. State of Related Art
Chlorine-free heat transfer fluids are desirable for use in refrigeration and air conditioning systems, because their escape into the atmosphere causes less damage to the environment than the currently most commonly used chlorofluorocarbon heat transfer fluids, such as trichlorofluoromethane and dichlorodifluoromethane. The widespread commercial use of chlorine-free refrigerant heat transfer fluids has been hindered, however, by the lack of commercially adequate lubricants. This is particularly true for one of the most desirable heat transfer fluids, 1,1,1,2-tetrafluoroethane, commonly known in the art as "Refrigerant 134a" or simply "R134a". Other fluoro-substituted ethanes are also effective heat transfer fluids.
The following patents and published patent applications disclose a number of general classes and specific examples of polyol esters as useful refrigerant lubricants with chlorine-free fluoro group-containing heat transfer fluids: U.S. Pat. No. 4,851,144; UK 2 216 541; U.S. Pat. No. 5,021,179; U.S. Pat. No. 5,096,606; WO 90/12849 (Lubrizol); EP 0 406 479 (Kyodo Oil); EP 0 430 657 (Ashai Denka KK); EP 0 435 253 (Nippon Oil); EP 0 445 610 and 0 445 611 (Hoechst AG); EP 0 449 406 (Tonen Corp.); EP 0 458 584 (Unichema Chemie BV); and EP 0 485 979 (Hitachi).
Refrigeration lubricants must have a wide variety of viscosities and other physical and chemical properties to achieve optimal effectiveness in the many different types of refrigeration equipment currently in use.
For example, conventional home refrigerators generally are designed to maintain cool temperatures within a relatively small and well insulated space. Accordingly, low power compressors are generally employed for such refrigerators, and lubricants with relatively low viscosities at normal operating temperatures are generally satisfactory for such use. Low viscosity lubricants are preferred for economy of operation, because with all other factors being equal, a refrigerant working fluid comprising a low viscosity lubricant reduces power consumption because of the greater ease of circulation of the lower viscosity refrigerant working fluids. With the rising cost of electric power, consumers are requiring greater efficiency in their home appliances.
By contrast, automobile air conditioners and industrial refrigeration systems require relatively higher viscosity lubricants by reason of the more extreme conditions under which such systems operate.
Even in low power compressors, a certain minimum viscosity is required to avoid excessive depletion of the lubricant from those surfaces of refrigeration machinery that need lubrication during operation, but are not immersed in the refrigerant working fluid, e.g., during periods in which the compressor and other moving parts of the refrigeration system are idle.
Mutual miscibility between the refrigerant and lubricant of the refrigerant working fluid is also very important, as the lubricant should not separate from the refrigerant at high or low temperatures. If the miscibility between the lubricant and refrigerant is low, moving parts of the refrigeration system may seize as a result of inadequate lubrication.
In addition, the lubricant for a refrigeration system must have high hydrolytic stability, must not be corrosive to the refrigerating machinery, and must not chemically react with the refrigerant over a wide temperature range. The hydrolytic stability of a refrigeration lubricant is extremely important. Ester lubricants that are not hydrolytically stable tend to react with moisture present in the compressor to form acidic substances that can lead to corrosion and otherwise interfere with the proper operation of the compressor. A related concern is the relative amount of unreacted acid in the lubricant. In this regard, lubricants containing an acid value of no more than 0.2 AV may cause corrosion problems or otherwise interfere with the effectiveness of the compressor. Accordingly, the ester lubricant base stocks should have an acid value of no greater than 0.2 AV as measured by the test for hydrolytic stability and preferably no greater than 0.1 AV, with increasing preference no greater than 0.08, 0.06 and 0.05, 0.04, 0.03, 0.02, and 0.01 AV.
Under the laws of many countries, any new chemical has to be tested for safety before it can legally be sold for general use, and any product obtained by reacting new mixtures of ingredients is usually defined as a new chemical. Therefore, every distinct mixture of polyol esters is likely to be subject to regulatory approval at very substantial expense for each such distinct mixture, in order to be legally sold in any country where such laws are in effect. One such regulatory system is the European EINCES registry. Thus, in formulating a lubricant for refrigeration applications, it is advisable to use known chemical entities whose properties are well characterized, and especially those that have been previously approved by governmental regulatory agencies, as such chemicals in different formulations are often exempted from legal requirements for regulatory approval, or at least subjected to less extensive, and therefore less expensive, testing than is required for mixtures legally categorized as new chemical entities. There is, therefore, a technical and economic incentive to provide lubricant base stocks suitable for most or all of the wide variety of conditions of lubricant service by blending from the smallest possible number of chemical components.
Accordingly, it is an object of the present invention to provide a lubricating oil for a chlorine-free refrigerant, and in particular 1,1,1,2-tetrafluoroethane refrigerant, that is miscible, but unreactive, with the refrigerant over a wide temperature range. Another object of the invention is to provide a lubricant having a controlled range of viscosity values, as well as satisfactory hydrolytic stability and very low concentration of unreacted acid. A further object of the present invention is to prepare a lubricant from registered chemicals or blends of chemicals that are approved for commercial use, or require little effort and expense to obtain pre-marketing approval.